The present invention relates to a radiological imaging apparatus using radiations, and particularly relates to a radiological imaging apparatus, such as a positron emission computed tomographic apparatus (hereinafter referred to as “PET” apparatus), suitable for performing a radiographic inspection, and a cooling method of the radiological imaging apparatus.
An inspection technique using radiations can nondestructively inspect the inside of a testing subject. Particularly, radiographic inspection techniques for human bodies include X-ray CTs, PETs, single photon emission computed tomographic apparatuses (hereinafter referred to as “SPECT” apparatus) and the like.
Any of these techniques is a technique in which the physical quantity of an inspection object is measured as an integral value in a radiation traveling direction, and the integral value is back-projected, whereby the physical quantity of each voxel in the testing subject is calculated to form an image. In these techniques, it is necessary to process an enormous amount of data, and high-speed and detailed images have been provided with rapid development of computer technologies in recent years.
The PET and SPECT as radiological imaging apparatuses are methods capable of detecting functions and metabolism at a molecular biological level which cannot be detected by an X-ray CT or the like, and can provide functional images of a body. The PET is a method in which a radioactive agent labeled with a positron emission nuclear species such as 18F, 15O or 11C and the distribution thereof is measured to form an image. The agents include fluoro-deoxy-glucose (2-[F-18]fluoro-2-deoxy-D-glucose, 18FDG) and the like, and such an agent is used for identification of a tumor site making use of the fact that the agent is highly accumulated in a tumor tissue due to saccharometabolism.
A radiation nuclear species incorporated in the body decays to emit a positron (β+). The emitted positron emits a pair of annihilation γ-rays (annihilation γ-ray pair) each having energy of 511 keV when bonding to an electron to annihilate. Because this annihilation γ-ray pair is emitted in approximately opposite directions (180±0.6 degrees), projection data can be obtained by detecting the annihilation γ-ray pair at a time by a plurality of radiation detectors placed to surround the testing subject, and accumulating data in their emission directions. By back-projecting projection data (using a filtered back projection method or the like), an emission position (position at which the radiation nuclear species is accumulated) can be specified to form an image.
The SPECT is a method in which a radioactive agent labeled with a single photon emission nuclear species is administered, and its distribution is measured to form an image. A single γ-ray having energy of about 100 keV is emitted from the agent, and this single γ-ray is measured by the radiation detector. In measurement of the single γ-ray, the traveling direction of thereof can not be identified, and therefore in the SPECT, projection data is obtained by inserting a collimater in the front face of the radiation detector and detecting only a γ-ray from a specified direction. As in the PET, image data is obtained by back-projecting projection data using the filtered back projection method and the like. The SPECT is different from the PET in that no coincidence measurement is necessary due to measurement of a single γ-ray, and thus the number of radiation detectors is small, and so on, and the apparatus configuration is simple.
In the above described conventional radiological imaging apparatuses of the PET, the SPECT and the like, a scintillator is used as the radiation detector. The scintillator temporarily converts an incident γ-ray into visible light and then reconverts the visible light into an electric signal by a photomultiplier (photomul). The scintillator has a disadvantage that its energy resolution is low and an accurate diagnosis cannot necessarily be performed because the number of photons generated during conversion into visible light is small, and in addition, two-stage conversion processes are required as described above. Particularly, the deterioration in energy resolution is a cause of impossibility of quantitative evaluation during 3D imaging in the PET. That is because an energy threshold of the γ-ray must be reduced due to the low energy resolution, and body-interior scattering as noises increasing during 3D imaging is detected in a large amount.
Thus, in recent years, attention has been given to use of a semiconductor detector as the radiation detector for the radiological imaging apparatus. The semiconductor detector converts an incident γ-ray directly into an electric signal, and has a characteristic of a high energy resolution because of a large number of generated electrons and hole pairs.
Usually, characteristics such as a time resolution and an energy resolution in the scintillator and the semiconductor detector are known to deteriorate under a high-temperature environment, and as a measure against this, a radiological imaging apparatus comprising a cooling mechanism has been disclosed (see, for example, JP-A-10-160847 (all pages) and JP-A-9-276262 (all pages)
In the PET inspection, it is necessary to make a determination on coincidence (coincidence measurement) of detected events for detecting an annihilation γ-ray pair. Because fluctuations occur due to noises and the like of a radiation detector and a circuit system at a time of detection of the annihilation γ-ray pair, an acceptable specific coincidence time window is provided for making a determination on coincidence, and a determination is made based on the premise that two events detected in this coincidence time window are coincident.
For improvement of image quality and improvement of quantifiability of image information in the radiological imaging apparatus, characteristics of the time resolution and the energy resolution in the above described scintillator and semiconductor detector are improved.
If the characteristic of the time resolution is improved, the above described coincidence time window can be shortened. As a result, the probability of accidentally capturing γ-rays which are not a real annihilation γ-ray pair is reduced. The accidentally captured γ-ray pair (accidental coincidence event) does not retain real positional information, and therefore such noise components are eliminated, whereby image quality and quantifiability of image information are improved. If the characteristic of the energy resolution is improved, the γ-rays by body-interior scattering can be eliminated, and thus image quality and quantifiability of image information are improved.
However, in a situation in which with enhancement of the performance of the radiological imaging apparatus, the number and the density of radiation detectors are being increased, and with downsizing of the apparatus, the denseness of electronic circuit devices and the like incorporated therein is being increased, there are concerns that even if the above described conventional cooling mechanism is applied, heat generated from the electronic circuit device (signal processing circuit) including the radiation detector cannot sufficiently be cooled, and as a result, the characteristics of the time resolution and the energy resolution are deteriorated.
An object of the present invention is to provide a radiological imaging apparatus which can inhibit transmission of heat generated in the signal processing circuit to the radiation detector, improve the time resolution and the energy resolution, and perform an accurate diagnosis, and a cooling method of the radiological imaging apparatus.